A&A Supplement series, Vol. 121, February 1997, 213 - 222
Received September 13, 1995; accepted May 22, 1996
Spectroscopy of the open cluster Blanco 1
II. H emission as an indicator of relative age
P.M. Panagi - M.A. O'Dell
Send offprint request: P.M. Panagi
Joint Center for Mesoscale Meteorology, Universty of Reading, Reading, UK
Visiting Fellow, Astronomy Centre, Univ. Sussex, Falmer, Brighton, BN1 9QH, UK
Dept. Physics & Astronomy, Univ. St. Andrews, North Haugh, Fife, KY16 9SS, Scotland
We present the results of H(6562Å) and Li I(6708Å) observations of 114 low-mass stars of the young open cluster Blanco 1. We also present observations of 30 stars in Ca II(K). This work extends the first Blanco 1 spectroscopic study of Panagi et al. (1994).
From a sample of four well-studied clusters, including Blanco 1, we find that the fraction of H emission-line stars amongst K dwarfs is a good indicator of relative age, with a smaller fraction indicative of older age. Blanco 1 shows a relatively small fraction of emitters, inconsistent with previous age estimates for the cluster. We estimate the cluster age to to be 90 25 Myr, slightly older than the Pleiades. The method is shown to be more sensitive to age than lithium and a useful alternative to other age measurement techniques.
The variation of H with (B-V) is similar to that observed in the older solar neighbourhood dwarfs, suggesting that, at least for the absorption-line stars, the contribution of stellar rotation to the equivalent width is unclear.
We combine both spectroscopy and photometry to revise cluster membership and give accurate positions for all these stars.
keywords: Galaxy: open clusters and associations: blanco 1 -- stars: activity -- stars: chromospheres -- stars: late-type